The present invention relates generally to apparatus for dispensing liquids, and more particularly, to apparatus which are capable of selectively delivering premeasured volumes of liquid.
Liquid dispensing devices are used for a variety of purposes in domestic, industrial, and scientific environments. A common type of liquid dispenser employs a plunger in a barrel where the liquid is forced out of a nozzle connected to the plunger as the plunger is depressed. Such plunger-type liquid dispensers are ubiquitous and employed to deliver a variety of substances, such as reagents, diluents, soaps, emollients, oils, and the like.
Of particular interest to the present invention are plunger-type liquid dispensers which have been adapted to provide delivery of premeasured liquid volumes where the precise volume may be selected by adjusting the dispenser mechanism in some manner. Usually, the dispensers are modified so that the stroke or travel of the plunger may be limited so that the delivered volume of liquid may be selected to be some particular percentage of the maximum delivery volume achieved with a full stroke of the plunger.
A variety of mechanisms for limiting the stroke or travel of a plunger in a plunger-type liquid dispenser has been proposed. Most simply, an adjustable collar or stop member may be placed on the plunger shaft in order to physically limit the available length of travel. Usually, calibration marks are placed on the shaft so that the collar or stop member may be aligned with the marks in order to provide a desired volumetric delivery. Such liquid dispensers are described in U.S. Pat. Nos. 3,653,556; 3,863,807; and 4,358,027. The '556 and '807 patents each disclose a movable collar placed on the external portion of the plunger shaft, while the '027 patent discloses a stop member formed internally to the plunger assembly.
While the use of an adjustable collar or stop member is a particularly straight forward mechanical solution to the problem of limiting plunger stroke length, it suffers from certain drawbacks. First, the stop members may be subject to slippage on the plunger shaft which can cause inaccurate volumetric delivery, particularly after a long period of use where the stop member has moved incrementally over time. Second, it can be difficult to precisely align the stop member with a calibration mark, particularly when the stop member is not disposed precisely horizontally relative to the vertical shaft. Third, when the stop member is internal to the plunger mechanism, disassembly of the liquid dispenser is necessary to adjust the liquid volume being delivered. Alternatively, when the stop member is external to the dispenser, it can be unsightly, collect dirt and debris, and be subject to accidental misalignment. Finally, stop members are often designed to be firmly attached to the plunger, e.g., using screws to hold them in place, which can make changing the volume delivery rate very inconvenient.
As an improvement over the use of collars and stop members for adjusting the volumetric delivery, it has been proposed to provide a plurality of channels on the plunger, where the channels are of different lengths. A stop member is then provided on the barrel or elsewhere on the dispenser, where the stop member can be rotated to travel in a preselected one of the channels in order to limit the relative stroke of the plunger to the barrel. This design is described in U.S. Pat. No. 4,081,111. Although an improvement over the use of collars and stop members, the design of the '111 patent still suffers from disadvantages. In particular, the channels on the shaft extend from the upper end downward so that the channels can be changed only when the plunger is in its fully depressed position relative to the barrel. This requires that the plunger be depressed through a discharge cycle before the volume can be changed, which can be wasteful. Second, the grooves in the plunger shaft extend upward out of the barrel when the plunger is in its upward position. This allows dirt and debris to gather in the grooves which can interfere with the operation of the dispenser and potentially carry contaminants into the barrel where the liquid is collected prior to delivery. Finally, the design disclosed in the '111 patent requires a separate external selector mechanism for rotating the stop member which travels in the channels formed on the plunger. Such a method of construction is relatively complex and expensive.
Other patents of interest include U.S. Pat. Nos. 3,485,090; 2,521,433; 2,123,737; 1,917,678; and 1,724,766.
For the above reasons, it would be desirable to provide plunger-type liquid dispensers which may be mechanically adjusted in order to provide for the precise delivery of different volumetric amounts of the liquid. It would be particularly desirable if the volume of liquid being delivered could be adjusted without disassembly of the dispenser and if the mechanism for adjusting the volumetric delivery could be fully enclosed within the dispenser. In particular, it would be desirable if the volumetric delivery rate could be adjusted while the plunger is in its upward or loaded position so that there would never be a need to discharge a liquid volume prior to selecting the needed volumetric delivery rate for the next dispensing cycle.